I would put 3 tangs in a 180, but as I said, I would want them small to start with, and even then I would do this knowing in the back of my mind that some day I will have to find homes for them. This isn't like fresh water where you can have 3 or 4 adult oscars in a 180 with no problem. These fish will feel the confinement eventually. If you want a purple tang, wait for a smaller one IMO.
Also, powder blues tend to be quite aggressive. Just a FYI.
+1 it's no fun giving this advice because everyone loves tangs. They can be kept very happy and fat, with never a sign of ICH, for years and years. No sterilizers. It's the difference in housing a fish or letting it prosper. But I've been tang police long enough. I can only leave the advice that if you really want to have a good home for these fish, just get a long tank with lots of racing around and hiding room. It makes a huge difference.
Are you saying that this takes the place of a large tank?
Sounds like an extremely elaborate description of a powerhead to me.
I have two wave-makers in my tank just FYI.
Yep...pretty much all day